Tens of thousands of pro-democracy protesters defied authorities to hold an unsanctioned march through Hong Kong on Sunday, a day after riot police fired rubber bullets and tear gas to disperse another illegal gathering, plunging the financial hub deeper into crisis.

Huge crowds made their way through the main island's streets in a now-familiar scene, but Sunday's march had a key difference -- police had banned it.

Officials had initially only given permission for a rally in a park in the commercial district known as Central.

But the crowds quickly spilled into the surrounding streets and began walking east in spite of the ban, ratcheting up the likelihood of renewed clashes.

"I feel so conflicted, seeing young people sacrifice their future for Hong Kong," a 22-year-old student protester called Marcus told AFP, bursting into tears.

The latest march comes a day after a town near the border with mainland China descended into chaos as police battled protesters holding another banned rally against suspected pro-government triad gangs who beat up democracy demonstrators there last weekend.

Riot police used tear gas throughout the afternoon and evening in Yuen Long after tense standoffs with protesters, some of whom were throwing projectiles and had surrounded a police van.

Rubber bullets were fired later in the clashes, which ended when officers baton-charged the last remaining demonstrators inside the town's metro station, leaving pools of blood in the same concourse where the suspected triads had attacked the previous weekend.

Saturday's violence compounds the political crisis engulfing the city's pro-Beijing leadership who are seemingly unable, or unwilling, to end the chaos.